This invention relates to a method for monitoring the dose of penetrating radiation absorbed by an object. It relates especially to a method of personnel monitoring wherein the dosimeter comprises a storage phosphor and wherein the result of the dosimetry is displayed on decentralized displays.
Several means and ways have been developed to monitor the amount of radiation that has been absorbed by an object. Especially in the field of personnel monitoring several types of dosimeters and readout apparatus have been proposed.
Well-known types of dosimeters are, e.g. based on CsI-crystal scintillators, mostly in the form of a pencil, which provide permanent control. When a quantified critical or threshold value becomes exceeded, a system in the form of a sound alarm may warn the controlled person. Another detection system makes use of detectors in the form of a badge which, after having been borne during a certain period of time are controlled centrally. Quantifying irradiation can be based on silver halide photography (as, e.g. in nuclear power stations, as described in xe2x80x9cGebrauchsanweisung fxc3xcr das Personendosimeter mit Ganzkxc3x6rperdosimetersonden, Typ GSF-Film-GD 10/20, GSF-Forschungs-zentrum fxc3xcr Umwelt und Gesundheit GmbHxe2x80x94Institut fxc3xcr Strahlenschutzxe2x80x94Auswertungsstelle fxc3xcr Strahlendosimeterxe2x80x94Stand: 1 Mxc3xa4rz 1994).
Another quantifying method can be based on thermoluminescence (e.g. with LiF detectors) or on PSL-dosimetry wherein phosphate glass becomes stimulated with a pulsed ultraviolet laser and wherein erasure is performed thermally.
In EP-A-844 497 a dosimeter using a storage phosphor as means for monitoring the absorbed does has been disclosed. The amount of energy of the penetrating radiation stored in the phosphor is proportional to the absorbed dose and can be read out and the remaining amount of energy stored in the phosphor can be erased by erasing radiation.
However, most of the means for personnel monitoring are not easily connected to an automatic reading system wherein not only the radiation dose absorbed on a particular moment can be read, but where also the radiation history of each individual person can easily be tracked. Moreover, most of the means for personnel monitoring have, for reading, to be processed in a centralized place and the result is not directly available, neither in time nor place, to the person being monitored. Therefore, there is still a need for a further method for personnel monitoring wherein a reusable dosimeter can be used that can automatically be read out, the result displayed in a decentralized way and where the radiation history of a person can automatically be tracked.
It is an object of the invention to provide a method for monitoring the dose of penetrating radiation absorbed by an object using a reusable dosimeter making it possible to automatically keep track of the xe2x80x9cradiation historyxe2x80x9d of the object.
It is a further object to provide a method for monitoring the dose of penetrating radiation absorbed by an object using a reusable dosimeter making it possible to check the absorbed dose in a decentralized reader.
Further objects and advantages of the invention will become clear from the detailed description hereinafter.
The objects of the invention are realized by providing a method for monitoring a dose of penetrating radiation absorbed by an object, comprising the steps of:
providing said object with a monitoring device having a storage phosphor for storing energy from penetrating radiation,
at predetermined intervals, coupling said monitoring device to a source of stimulating light in such a way that said stimulating light impinges on said phosphor,
activating said source of stimulating light so as to cause said storage phosphor to emit an amount of fluorescent light in proportion to an amount of stored energy,
reading said amount of fluorescent light and converting said amount of fluorescent light in an electric signal value,
storing electric signal value(s) obtained at said predetermined intervals and processing them so as to evaluate a total amount of radiation absorbed by said object,
comparing said total amount with a pre-defined threshold value for obtaining a difference, and
displaying said difference on a decentralized display.
It was found that dosimeters, for use in the dosimetry of objects as well as for use in personnel monitoring, wherein the radiation absorbing device contains a stimulable phosphor for absorbing the dose of penetrating radiation could easily be read out without the need of much processing. In such dosimeters, the amount of absorbed radiation could directly, in time as well as in place, be converted in an electric signal value that could be stored and manipulated in a computer. In EP-A-844 497, EP-A-892 283 and European Application 98203794, filed Nov. 10, 1998, dosimeters comprising stimulable phosphors have been disclosed.
This presents an advantage over the classical dosimeter. In the latter dosimeters, the absorbed dose of penetrating radiation is only known after some time and is measured away from the place where penetrating radiation is used. This can be harmful to the personnel working with penetrating radiation since such person is only warned of having received a dose of penetrating radiation only after some time. Thus, such a person can, in the meantime between receiving the radiation dose and knowing that dose, still work in an environment where the risk of radiation exists, although on the basis of the received dose, such person would be prohibited from working in such place.
Thus, a dosimetric method that makes it possible to read the absorbed dose immediately at the place where the radiation risk is present, gives the advantage that a person having received a dose of penetrating radiation can know it immediately, and the necessary safety measures can be taken directly.
This problem can be solved when using a photostimulable phosphor as a device for absorbing the penetrating radiation. Storage phosphors are inorganic substances that, upon irradiation by penetrating radiation, absorb energy of the penetrating radiation and store a portion of said energy. The stored energy can then later on be detected by irradiating said storage phosphor (stimulating said phosphor) by electromagnetic radiation with wavelengths ranging from 300 nm to 1200 nm (i.e. by stimulation light) or by heating said phosphor. Upon said irradiation or heating all or a portion of the energy stored in the storage phosphor is released as electromagnetic radiation (e.g., Ultraviolet (UV) light, visible light, and Infrared (IR) light). This electromagnetic radiation, further on called xe2x80x9cfluorescent light,xe2x80x9d can then be detected. Such phosphors are well known from their use in medical imaging, where, after exposure to penetrating radiation the phosphor is pixel-wise stimulated. When using such a phosphor in dosimetry, the phosphor does not have to be pixel-wise stimulated, a simple overall stimulation is sufficient, since in dosimetry only the amount of absorbed penetrating radiation has to be determined. Thus, the detection of an amount of penetrating radiation stored in a storage phosphor (photostimulable phosphor) can proceed without any complicated processing. Therefore, it is possible to build a simple, small, inexpensive reader for reading the amount of energy stored in the storage phosphor and displaying this amount. Such a reader can easily be placed at or near the location where the penetrating radiation is used so that the absorbed dose can immediately be read at the place where the radiation risk is present.
Basically the method of the present invention comprises the steps of
providing an object with a device for absorbing penetrating radiation, including a storage phosphor for storing energy from said penetrating radiation,
at predetermined intervals, coupling said storage phosphor to a source of stimulation light, in such a way that said stimulation light impinges on said phosphor,
activating said source of stimulation light so as to cause said storage phosphor to emit an amount of fluorescent light in proportion to an amount of stored energy,
reading said amount of fluorescent light and converting it into an electric signal value,
storing electric signal value(s) obtained at said predetermined intervals and processing them so as to evaluate a total amount of radiation absorbed by said object,
comparing said total amount with a pre-defined threshold value for obtaining a difference value, and
displaying said difference value on a decentralized display.
This decentralized display can be a display screen or a printer.